THE BRAIN abhors ambiguity, yet we are curiously attracted to it. Many famous visual illusions exploit ambiguity to titillate the senses. Resolving uncertainties creates a pleasant jolt in your brain, similar to the one you experience in the “Eureka!” moment of solving a problem. Such observations led German physicist, psychologist and ophthalmologist Hermann von Helmholtz to point out that perception has a good deal in common with intellectual problem solving. More recently, the idea has been revived and championed eloquently by neuropsychologist Richard L. Gregory of the University of Bristol in England.

So-called bistable figures, such as the mother-in-law/wife (a) and faces/vase (b) illusions, are often touted in textbooks as the prime example of how top-down influences (preexisting knowledge or expectations) from higher brain centers—where such perceptual tokens as “old” and “young” are encoded—can influence perception. Laypeople often take this to mean you can see anything you want to see, but this is nonsense—although, ironically, this view contains more truth than most of our colleagues would allow.